Simultaneous Removal of Particulate Matter and Gas-Phase Pollutants within Electrostatic Precipitators: Coupled In-Flight and Wall-Bounded Adsorption

Older combustion facilities are favored to comply with imposed regulations limiting emissions of trace toxic metals by modifying the operation of existing emissions control devices to concurrently capture the regulated toxic metal compound; a prominent example of this is adsorption of mercury by added activated carbon within electrostatic precipitators (ESPs). Our previous quasi-1-D analyses employed simplifying assumptions, some of which were initially proposed decades ago. The present study leverages a modern computational platform to remove restrictive, simplifying assumptions and capture details of multi-phase flows and electric and electro-hydrodynamic phenomena that were previously out of computational reach. The results reveal the conflicting interdependent trends in particulate matter (PM) collection and adsorption of a trace gas-phase pollutant, assumed here to be mercury. In particular, we show the power of the inverse correlation between PM removal efficiency and trace pollutant adsorption efficiency, for two representative particle size distributions for increasing treatment times and under constant electrical conditions (applied voltage and current density). Also noteworthy is the finding that the two mechanisms of trace pollutant adsorption within an ESP, in-flight adsorption by suspended sorbent particles and wall-bounded adsorption by sorbent-covered walls, cannot be treated as additive. Wall-bounded adsorption depletes trace pollutant concentrations in the concentration boundary layer near the walls, reducing the driving potential for in-flight adsorption in these regions. For the conditions simulated, the additional rate of pollutant removal by the former is almost entirely counter-balanced by diminished rates of pollutant removal by the latter. Such findings highlight the need to optimize emissions control processes originally designed for a single pollutant but operated for the purpose of dual-pollutant control.